For data storage devices such as hard disk drives, taking fly-height (FH) measurements in standard commercial fly-height testers (FHTs) typically requires to first measure optical constants, ie. reflective index (n) and extinction coefficient (k), by using separated equipment such as a spectroscopic ellipsometer.
One problem that may arise is that there is a difficulty to ensure that n and k are measured at the same spot for the ellipsometry measurement and for the FH testing performed after the ellipsometry measurement. Thus, due to potential variation in n and k across a slider surface, errors as large as 1.0 to 2.0 nm may be encountered in FH measurements. This error range is not acceptable since FHs are approaching the sub-10 nm regime due to increases in storage densities of modern hard disk drives.
In addition to the above, the n and k ambiguity typically leads to other problems when load/unload (L/UL) calibration is employed. L/UL calibration is currently recognised as an important step in commercial FH testing as described in U.S. Pat. No. 5,457,534. L/UL calibration is used to identify the maximum and minimum of a FH interferometric signal as a slider is lifted or unloaded from its flying position above a disk surface. Due to the design of current negative pressure sliders, it is typically very difficult to perform L/UL calibration at a slider trailing edge. Thus, if it is derived to measure the FH at the trailing edge, the L/UL calibration has to typically be performed elsewhere on the slider. However, because of the uncertainty in the n and k properties across the slider surface, it is typically not reliably possible to find positions of similar n and k for the L/UL calibration and generally, the FH testing spot.
Furthermore, an inspection of the standard equation for intensity, IA, for thin film interference (see eq. 1) reveals that two parameters dependent on the slider n and k properties are present.
                              I          A                =                                            r              A              2                        +                          r              2              2                        +                          2              ⁢                                                          ⁢                              r                A                            ⁢                              r                2                            ⁢              cos              ⁢                                                          ⁢              δ                                            1            +                                          r                A                2                            ⁢                              r                2                2                                      +                          2              ⁢                                                          ⁢                              r                A                            ⁢                              r                2                            ⁢              cos              ⁢                                                          ⁢              δ                                                          eq        .                                  ⁢        1            The two parameters are the reflectance at slider-air interface, rA and path difference, δ. δ is a function of fly height, h, incident wavelength, λ, and phase change on reflection, φa, where φs is n, k dependent. Therefore, yet another problem that may arise is that use of inaccurate n and k values results in an inaccurate calculation of the FH, based on interferometric intensity.
Thus, being unable to use similar and accurate n and k values at both the L/UL and FH testing points typically results in a double penalty.
In an attempt to solve the above problems, in-situ n and k compensation has been suggested. In-situ n and k compensation uses a means through which, or a parameter by which, to predict, or measure the local optical properties, namely n and k.
An in-situ n and k compensation method described in IEEE Trans. Magn., 34(2), 459, 1998 by Womack et al. is directed at gauging n and k by means of intensity, where an empirical linear relationship generally exists between calculated reflectivity and ellipsometric measured n and k properties. In Womack et al.'s approach, a glass reference prism is used next to a spinning glass disk to allow a separate measurement of the reflectivity R. However, this direct approach to link R to n and k may give rise to a problem in that the obtained measured intensity contains a component mediated by the interferometric nature of the approach and therefore, does not compute as the intensity that would be obtained by calculations that use the ellipsometrically measured n and k values. It has been observed that with inherent FH information mixed with n and k induced intensity changes, it may not be possible to accurately determine n and k directly by using Womack et al.'s approach. It has been found that the measured intensity is approximately 20% lower than the predicted value based on the n and k model, indicating that the Womack et al.'s direct approach may not be applicable.
Hence, there exists a need for a method and apparatus for measuring FH to address at least one of the above problems.